Probabilistic Models. Models describe how (a portion of) the world works

Transcription

1 Probabilistic Models Models describe how (a portion of) the world works Models are always simplifications May not account for every variable May not account for all interactions between variables All models are wrong; but some are useful. George E. P. Box What do we do with probabilistic models? We (or our agents) need to reason about unknown variables, given evidence Example: explanation (diagnostic reasoning) Example: prediction (causal reasoning) Example: value of information 1

2 Bayes Nets [These slides were created by Dan Klein and Pieter Abbeel for CS188 Intro to AI at UC Berkeley. All CS188 materials are available at

3 Independence 3

4 Example: Independence N fair, independent coin flips: H 0.5 T 0.5 H 0.5 T 0.5 H 0.5 T 0.5 4

5 Conditional Independence P(Toothache, Cavity, Catch) If I have a cavity, the probability that the probe catches in it doesn't depend on whether I have a toothache: P(+catch +toothache, +cavity) = P(+catch +cavity) The same independence holds if I don t have a cavity: P(+catch +toothache, -cavity) = P(+catch -cavity) Catch is conditionally independent of Toothache given Cavity: P(Catch Toothache, Cavity) = P(Catch Cavity) Equivalent statements: P(Toothache Catch, Cavity) = P(Toothache Cavity) P(Toothache, Catch Cavity) = P(Toothache Cavity) P(Catch Cavity) One can be derived from the other easily 5

6 Conditional Independence Unconditional (absolute) independence very rare (why?) Conditional independence is our most basic and robust form of knowledge about uncertain environments. DEF: X is conditionally independent of Y given Z if and only if: or, equivalently, if and only if 6

7 Conditional Independence What about this domain: R Traffic Umbrella Raining U T R U T u, t, r P(u, t r) = P(u r) P(t r) 7

8 Conditional Independence What about this domain: Fire Smoke Alarm F S A A F S P(a s, f) = P(a s) 8

9 Conditional Independence and the Chain Rule Chain rule: Trivial decomposition: With assumption of conditional independence: Bayes nets / graphical models help us express conditional independence assumptions 9

10 Ghostbusters Chain Rule Each sensor depends only on where the ghost is P(T,B,G) = P(G) P(T G) P(B G) e.g. (.5) (.8) (.4) =.16 That means, the two sensors are conditionally independent, given the ghost position T: Top square is red B: Bottom square is red G: Ghost is in the top Givens: P( +g ) = 0.5 P( -g ) = 0.5 P( +t +g ) = 0.8 P( +t -g ) = 0.4 P( +b +g ) = 0.4 P( +b -g ) = 0.8 T B G P(T,B,G) +t +b +g t +b -g t -b +g t -b -g t +b +g t +b -g t -b +g t -b -g

11 Bayes Nets: Big Picture 11

12 Bayes Nets: Big Picture Two problems with using full joint distribution tables as our probabilistic models: Unless there are only a few variables, the joint is WAY too big to represent explicitly Hard to learn (estimate) anything empirically about more than a few variables at a time Bayes nets: a technique for describing complex joint distributions (models) using simple, local distributions (conditional probabilities) More properly called graphical models We describe how variables locally interact Local interactions chain together to give global, indirect interactions For about 10 min, we ll be vague about how these interactions are specified 12

13 Example Bayes Net: Insurance What determines Liability Cost? What factor(s) determine whether there is going to be an Accident? 13

14 Example Bayes Net: Car If the car won t start, what is the cause? 14

15 Graphical Model Notation Nodes: variables (with domains) Can be assigned (observed) or unassigned (unobserved) Arcs: interactions Similar to CSP constraints Indicate direct influence between variables Formally: encode conditional independence (more later) For now: imagine that arrows mean direct causation (in general, they don t!) 15

16 Example: Coin Flips N independent coin flips X 1 X 2 X n No interactions between variables: absolute independence 16

17 Example: Traffic Variables: R: It rains T: There is traffic Model 1: independence R Model 2: rain causes traffic R T T Why is an agent using model 2 better? 17 It can plan better.

18 Example: Traffic II Let s build a causal graphical model! Variables T: Traffic R: It rains L: Low pressure D: Roof drips B: Ballgame C: Cavity T R B L D C 18

19 Example: Alarm Network Variables B: Burglary A: Alarm goes off M: Mary calls J: John calls E: Earthquake! B E A J M 19

20 Bayes Net Semantics 20

21 Bayes Net Semantics A set of nodes, one per variable X A directed, acyclic graph A conditional distribution for each node A 1 A n A collection of distributions over X, one for each combination of parents values X CPT: conditional probability table Description of a noisy causal process A Bayes net = Topology (graph) + Local Conditional Probabilities 21

22 Probabilities in BNs Bayes nets implicitly encode joint distributions As a product of local conditional distributions To see what probability a BN gives to a full assignment, multiply all the relevant conditionals together: Example: = P(+cavity)P(-toothache +cavity)p(+catch +cavity) 22

23 Probabilities in BNs Why are we guaranteed that setting results in a proper joint distribution? Chain rule (valid for all distributions): Assume conditional independences: Consequence: Not every BN can represent every joint distribution The topology enforces certain conditional independencies 23

24 Example: Coin Flips X 1 X 2 X n h 0.5 t 0.5 h 0.5 t 0.5 h 0.5 t 0.5 Only distributions whose variables are absolutely independent can be represented by a Bayes net with no arcs. 24

25 Example: Traffic R +r 1/4 -r 3/4 P(-t +r) * P(+r) ¼ * ¼ 1/16 T +r +t 3/4 -t 1/4 -r +t 1/2 -t 1/2 25

26 Example: Alarm Network B P(B) +b Burglary Earthqk E P(E) +e b e What is P(b, e,a,j, a)? John calls A J P(J A) +a +j 0.9 +a -j 0.1 -a +j a -j 0.95 Alarm Mary calls A M P(M A) +a +m 0.7 +a -m 0.3 -a +m 0.01 B E A P(A B,E) +b +e +a b +e -a b -e +a b -e -a b +e +a b +e -a b -e +a a -m b -e -a

27 Bayes Nets So far: how a Bayes net encodes a joint distribution Next: how to answer queries about that distribution 27

28 Example: Traffic Causal direction R +r 1/4 -r 3/4 How do we get the joint distribution? +r +t 3/16 P(T,R) = P(R)P(T R) T +r +t 3/4 -t 1/4 -r +t 1/2 +r -t 1/16 -r +t 6/16 -r -t 6/16 +r +t = (3/4) (1/4) = 3/16 -r +t = (1/2) (3/4) = 6/16 -t 1/2 28

29 Example: Reverse Traffic Reverse causality? T +t 9/16 -t 7/16 R +t +r 1/3 -r 2/3 -t +r 1/7 +r +t 3/16 +r -t 1/16 -r +t 6/16 -r -t 6/16 Bayes Rule P(R T) α P(T R) P(R) -r 6/7 29

30 Causality? When Bayes nets reflect the true causal patterns: Often simpler (nodes have fewer parents) Often easier to think about Often easier to elicit from experts BNs need not actually be causal Sometimes no causal net exists over the domain (especially if variables are missing) E.g. consider the variables Traffic and Drips End up with arrows that reflect correlation, not causation What do the arrows really mean? Topology may happen to encode causal structure Topology really encodes conditional independence 30

31 Bayes Net Semantics A directed, acyclic graph, one node per random variable A conditional probability table (CPT) for each node A collection of distributions over X, one for each combination of parents values Bayes nets implicitly encode joint distributions As a product of local conditional distributions To see what probability a BN gives to a full assignment, multiply all the relevant conditionals together: 31

32 Size of a Bayes Net How big is a joint distribution over N Boolean variables? 2 N How big is an N-node net if nodes have up to k parents? O(N * 2 k+1 ) Both give you the power to calculate BNs: Huge space savings! Also easier to elicit local CPTs Also faster to answer queries (coming) 32

33 Conditional Independence X and Y are independent if X and Y are conditionally independent given Z (Conditional) independence is a property of a distribution Example: 33

34 Bayes Nets: Assumptions Assumptions we are required to make to define the Bayes net when given the graph: Beyond above chain rule Bayes net conditional independence assumptions Often additional conditional independences They can be read off the graph Important for modeling: understand assumptions made when choosing a Bayes net graph 34

35 Example X Y Z W Conditional independence assumptions directly from simplifications in chain rule: X Z Y Y W Z Additional implied conditional independence assumptions? 35

36 Independence in a BN Important question about a BN: Are two nodes independent given certain evidence? If yes, can prove using algebra (tedious in general) If no, can prove with a counter example Example: X Y Z Question: are X and Z necessarily independent? Answer: no. Example: low pressure causes rain, which causes traffic. X can influence Z, Z can influence X (via Y) Addendum: they could be independent: how? 36

37 This leades to the concept of D-separation Study independence properties for triples Analyze complex cases in terms of member triples D-separation: a condition / algorithm for answering such queries 37

38 Causal Chains This configuration is a causal chain X Y Z Guaranteed X independent of Z? No! One example set of CPTs for which X is not independent of Z is sufficient to show this independence is not guaranteed. Example: X: Low pressure Y: Rain Z: Traffic Low pressure causes rain causes traffic, high pressure causes no rain causes no traffic (Something else could cause traffic.) 38

39 Causal Chains This configuration is a causal chain Guaranteed X independent of Z given Y? Proof: X: Low pressure Y: Rain Z: Traffic Yes! Evidence along the chain blocks the influence 39

40 Common Cause This configuration is a common cause Y: Project due Guaranteed X independent of Z? No! One example set of CPTs for which X is not independent of Z is sufficient to show this independence is not guaranteed. Example: Project due causes both forums busy and lab full X: Forums busy Z: Lab full 40

41 Common Cause This configuration is a common cause Guaranteed X and Z independent given Y? Proof Y: Project due X: Forums busy Z: Lab full Yes! Observing the cause blocks influence between effects. 41

42 Common Effect Last configuration: two causes of one effect (v-structures) X: Raining Y: Ballgame Are X and Y independent? Yes: the ballgame and the rain cause traffic, but they are not correlated Still need to prove they must be (try it!) Are X and Y independent given Z? No: seeing traffic puts the rain and the ballgame in competition as explanation. This is backwards from the other cases Z: Traffic Observing an effect activates influence between possible causes. 42

43 The General Case 43

44 The General Case General question: in a given BN, are two variables independent (given evidence)? Solution: analyze the graph Any complex example can be broken into repetitions of the three canonical cases 44

45 D-Separation The Markov blanket of a node is the set of nodes consisting of its parents, its children, and any other parents of its children. d-separation means directional separation and refers to two nodes in a network. Let P be a trail (path, but ignore the directions) from node u to v. P is d-separated by set of nodes Z iff one of the following holds: 1. P contains a chain, u m v such that m is in Z 2. P contains a fork, u m v such that m is in Z 3. P contains an inverted fork, u m v such that m is not in Z and none of its descendants are in Z. 45

46 Active / Inactive Paths Gray nodes are observed. (the givens) Question: Are X and Y conditionally independent given evidence variables {Z}? Yes, if X and Y are d-separated by Z Consider all (undirected) paths from X to Y No active paths = independence! Active Triples not c-independent X Z Y (X Z Y) X Z Inactive Triples X Y Z X Z Y X Y Z X Z Y X Y Z X Z Y is not observed 46 Y

47 Active / Inactive Paths gray nodes are observed. Question: Are X and Y conditionally independent given evidence variables {Z}? We want NO ACTIVE PATHS. A path is active if each triple is active: Active Triples not c-independent Inactive Triples Causal chain A B C where B is unobserved (either direction) Common cause A B C where B is unobserved Common effect (aka v-structure) A B C where B or one of its descendants is observed 47

48 Active / Inactive Paths gray nodes are observed. Question: Are X and Y conditionally independent given evidence variables {Z}? Yes, if X and Y are d-separated by Z Consider all (undirected) paths from X to Y No active paths = independence! Active Triples not c-independent Inactive Triples A path is active if each triple is active: Causal chain A B C where B is unobserved (either direction) Common cause A B C where B is unobserved Common effect (aka v-structure) A B C where B or one of its descendents is observed All it takes to block a path is a single inactive segment If all paths are blocked by at least one inactive tuple, then conditionally independent. 48

49 D-Separation Query: Check all (undirected!) paths between? and If one or more active, then independence not guaranteed Otherwise (i.e. if all paths are inactive), then independence is guaranteed 49

50 Example Yes No R B No T T 50

51 Example L Inactive triple blocks whole path. Yes Yes R B No No L D T Yes L->R->T is inactive R->T<-B is active One inactive blocks the path R T B 51 T

52 Example Variables: R: Raining T: Traffic D: Roof drips S: I m sad Questions: T R S D No Yes No one active path, and one inactive path 52

53 Structure Implications Given a Bayes net structure, can run d- separation algorithm to build a complete list of conditional independences that are necessarily true of the form This list determines the set of probability distributions that can be represented 53

54 Bayes Nets Representation Summary Bayes nets compactly encode joint distributions Guaranteed independencies of distributions can be deduced from BN graph structure D-separation gives precise conditional independence guarantees from graph alone A Bayes net s joint distribution may have further (conditional) independence that is not detectable until you inspect its specific distribution 54